Method of treating brake drums



METHOD OF TREATING BRAKE DRUMS Filed March 19, 1935 MAJ INVENTOR. J ,q ggza HU/VT 445567 saw/A7256 F/g-T 4. BY

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Patented Nov. 24, 1936 PATENT oFFIcE METHOD OF TREATING BRAKE DRUMS J Harold Hunt and Albert P. Schweizer, Lansing, Mich., assignors to Motor Wheel Corporation, Lansing, Mich a corporation of Michigan Application March 19, 1935, Serial No. 11,882

4 Claims.

This invention. relates to the method of producing steel brake drums having substantially score resistant braking surfaces.

One of the most important parts of a brake 5' drum is its braking surface. It is generally conceded that certain forms of cast iron constitute the best braking surface for brake drums because of its high score resisting properties and because of its relatively high coefiicient of friction.

- However, cast iron cannot be formed or worked factory scor'e resisting properties when used as' the braking surface for brake drums. When so used there is a tendency for small particles of the steel, due it is believed to the extremely high tem- 25 peratures developed in the drum, to be deposited by the braking surface in the contacting friction element (lining). These small pieces of steel become embedded in the lining with the result that continued use of the brake drum scores or 36. tears the braking surface and ultimately impair the operation of the brake.

Heretofore in order to increase the score resisting properties of steel it has been necessary to,

heat the same above its critical point, quench 35 rapidly, and then reheat to a temperature between 600 and 800 F. When so treated the Brinell hardness of the steel is increased to 300 or more, its coefficient of friction materially decreased, and its ductility adversely affected to the 40 extent that it is substantially impossible to shape or form the same by cold die-drawing operations. It is also well known that metal of a Brinell hardness of 300 is practically non-machinable.

We have discovered that alloy steel containing 45 substantially one per cent (1%) or more manganese can be. given a simple treatment which increases its score resisting properties as a braking surface without materially decreasing its coefllj cient of friction or ductility, and without increas- 50 ing its hardness beyond approximately 200 Brinell. This particular alloy steel is herein referred to as high manganese steel.

High manganese steel, as received from the mill, has a satisfactory coefiicient of friction, is

66 reasonably ductile, and has a hardness of approximately 200 Brinell or slightly less. While it is somewhat more score resistant than unalloyed steel, it is not entirely free from the tendency to score previously described in connection with steel braking surfaces. It is subject, in a lesser 5 degree, to the same deterioration and ultimate invoperativenss as other steel.

The process of treating high manganese steel to increase its score resisting properties without decreasing its coefficient of friction or ductility 10 and without increasing its hardness beyond approximately 200 Brinell consists in heating it to a temperature of about 1550 F. and giving it a rapid air quench in air at atmospheric temperature. Micrographs of the steel before and after the treatment indicate that this treatment changes the crystalline structure of the steel from one containing substantially segregated areas of pearlite and ferrite to one in which there is substantially no such segregated-areas.

In the accompanying drawing:

Figure 1 is a reproduction of a micrograph of 100 magnification showing the crystalline structure of alloy steel having a manganese content of 1.65% and a carbon content of .22% before be-' fore being treated as just described;

Figure 2 is a reproduction of a micrograph of 100 magnification showing the crystalline strimture of the same steel after having been heated to a temperature of 1550 F. and cooled in air at atmospheric temperature; and

Figures 3, 4,-and 5 illustrate the method of manufacturing a brake drum.

By reference to Figure 1 it will be observed that I the pearlite (black portions) and ferrite (white portions) are arranged in segregated areas giving a banded appearance to the structure as a whole. By reference to Figure 2 it will be observed that the segregated areas of pearlite and ferrite have been substantially eliminated, that 40 each is blended into the other with' no perceptible line of demarkation between the two, and that some carbides are produced. It will be further observed that this steel is hypo-eutectoid steel.

After the treatment just described the high 5 manganese steel has substantially the same coeflicient of friction, ductility and hardness as before the treatment but its score resisting and wear resisting properties are materially increased. When the high manganese steel thus treated is used as a braking surface in brake drums its score resisting properties aresubstantially as great as the best form of cast iron and its wear resisting properties somewhat better than cast iron. I 56 .to improve its score resisting properties is readily adapted to inclusion as a step in the manufacture of brake drums. The manufacture of a composite brake drum is illustrated in Figures 3, 4 and 5.

A strip of high manganese steel ill, in the form in which it is received from the mill (see Fig. 1), is cut to the desired shape and size as illustrated in Figure 3. This strip is then coiled or rolled into a hoop formation as shown in Figure 4 -and the ends fastened together preferably by welding as indicated at H. The hoop thus formed is next heated to a temperature of about 1550 F. and allowed to cool quickly in air at atmospheric temperature, producing the crystalline structure shown in Figure 2.

The hoop is next sized to bring its inner (brak ing) surface to a truly concentric form. This is preferably accomplished by upsetting, or it may be done by expanding, or by a combination of the two. After the hoop ID has been sized a web I2 is attached thereto in a conventional manner as by means of the rivets l3.

When a brake drum is produced from ordinary steel such as SAE 1025, having less than 1% manganese, in accordance with the method just described, its score resisting properties are not materially improved. Such a drum will not withstand severe usage without deterioration. To increase the score resisting properties of such a drum, it is necessary, as previously explained, to heat it above its critical temperature, cool it, and then reheat it if necessary. When subjected to such a treatment as will materially increase its score resisting properties the hardness of the ordinary steel drum is increased to approximately 300 Brinell and consequently it is either impossible orimpractical to size the same by any of the standard sizing operations. In other words, a substantially score resistant braking surface can be produced by the simple method described when steel containing between approximately 1% and 2% of manganese is used withoutadversely affecting the ability to size the same after such treatment, whereas, the score resisting properties of ordinary steel are not materially improved by subjecting it to such treatment and when subjected to a treatment which will materially improve its score resisting properties, its ductility and machinability is adversely affected to the extent that it becomes practically impossible to size the finished brake drum.

A most striking and distinctive characteristic of the drum formed as just described is its performance under severe usage. Brake drums in use heretofore, including those having cast iron braking surfaces, tend to deteriorate when subjected to prolonged frictional contact. or to a relatively large number of severe frictional contacts at very brief intervals. The drum embodying and made in accordance with this invention does not deteriorate under such conditions but maintains both its score resisting and wear resisting qualities as well as its coefficient of friction. It is in effect a self conditioning brake drum.

While only the preferred embodiment and method of practicing the invention have been illustrated and described; it should be understood that it is not limited to such preferred method and embodiment, but is coextensive with the scope of the appended claims.

What is claimed is:

1. The method of producing steel brake drums having high score resisting properties which comprises providing an annular braking surface formed of hypo-eutectoid steel containing between approximately 1% and 2% of manganese, heating the braking surface to a temperature of approximately 1550 F., cooling the braking surface at a rate equivalent to cooling in air at atmospheric temperature, and thereafter sizing the braking surface to produce true circularity.

2. The method of producing steel brake drums having high score resisting properties which comprises providing an annular braking surface formed of hypo-eutectoid steel containing between approximately 1% a'nd 2% of manganese, heating the braking surface to a temperature of approximately 1550 F., cooling the braking surface at a rate equivalent to cooling in air at atmospheric temperature, and sizing the braking surface after it has cooled to produce true circularity.

3. The method of producing steel brake drums having high score resisting properties which comprises forming an annular braking surface of hypo-eutectoid steel containing between approximately 1% and 2% of manganese, heating the steel to a temperature of approximately 1550 F., and cooling the drum in air at atmospheric temperature.

4. The method of producing steel brake drums suitable for attaching to a drum back and having high score resisting properties which comprises providing a strip of hypo-eutectoid steel containing between approximately 1% and 2% of manganese, forming the strip into an annular braking surface,heating the steel to a temperature of approximately 1550 F., and cooling said strip, at a rate equivalent to cooling in air at atmospheric temperature.

J HAROLD HUNT. ALBERT P. SCHWEIZER. 

